I believe it is a reasonable hypothesis that if payroll taxes were removed, 2nd order effect would be that employers have more money to offer for all positions, and in a market driven job market, prices would increase and thus salaries would converge to X+Y, yet they would be worth the same as X today.

Yes, likely not exactly the same (a bit more kept by employers in overcrowded job markets, a bit less in others), but it would essentially support the interpretation that most of that is really a tax that goes out of employee "budget", or their total comp.